1. Field of the Invention
The present invention relates to a tomographic image obtainment apparatus and method for obtaining a tomographic image by radiography.
2. Description of the Related Art
In recent years, tomosynthesis radiography (tomosynthesis) has also been proposed in the field of X-ray radiography apparatuses (CR: computed radiography) to observe diseased parts in more detail. In tomosynthesis, an X-ray tube is moved and the subject is irradiated with X rays at various angles to obtain radiographic images of the subject. Then, the obtained radiographic images are added together to obtain a tomographic image (a cross-sectional image or a slice image) in which the state of the subject on a desired cross-sectional plane of the subject is emphasized.
In tomosynthesis radiography, a subject is radiographed at various angles of radiography (photography) by moving an X-ray tube parallel to a detector, such as a flat panel, or by moving the X-ray tube in a circle or in an ellipse. Accordingly, a plurality of radiographic images are obtained, and a tomographic image is reconstructed from the plurality of radiographic images. The tomographic image may be obtained by moving the plurality of radiographic images parallel to each other, by adjusting the sizes of the radiographic images and by adding the radiographic images together. Further, the radiographic images are obtained, as digital images, using digital-type solid-state detectors arranged in matrix form so that operations among the radiographic images are easily performed.
When a subject is irradiated with radiation a plurality of times to perform tomosynthesis radiography, if the dose of each radiation is the same as that of radiation in ordinary radiography, the subject is exposed to a large dose of radiation in total. Therefore, when the subject is irradiated with radiation a plurality of times, the dose of each radiation is reduced as the number of times of radiation increases. However, the specification of a detector that is used in an X-ray radiography apparatus is determined so that high quality images are obtained when radiography is performed at high dose values by assuming that the detector is used for ordinary X-ray radiography. Therefore, if radiographic images are obtained at extremely low doses, artifacts tend to appear. The artifacts have stripe patterns extending in a direction vertical or horizontal to the arrangement direction of the digital-type solid-state detectors, which are arranged in matrix form.
When a tomographic image is obtained by tomosynthesis, radiographic images are moved parallel to each other or the sizes of the radiographic images are adjusted. Further, the radiographic images are added to obtain the tomographic image. However, if the radiographic images are obtained by a digital-type solid-state area detector that is set parallel to the direction of the X-axis of a bed (table) for placing a subject and the obtained radiographic images are added, the aforementioned artifacts, which have the vertical or horizontal stripe patterns, appear at the same position and superposed one on another in some cases. In such cases, the artifacts are emphasized.
Therefore, a method for preventing vertical or horizontal stripe-shaped detector defects from appearing at the same position has been proposed (for example, U.S. Pat. No. 6,341,156 or the like). In this method, formation of artifacts is prevented by inclining the detector by angle α with respect to the direction of the X-axis of a bed (table) on which a subject is placed.
Further, when radiographic images are obtained, a tube voltage and a tube electric current (tube current) are adjusted to obtain radiographic images that have appropriate contrast. However, since an appropriate tube voltage and an appropriate tube current differ depending on the size of a subject, setting is changed for each patient. Further, it is desirable that the tube voltage and the tube current are also appropriately adjusted when radiography is performed using a CT (computed tomography apparatus) for obtaining tomographic images. Therefore, a method for setting, based on the transmittance of radiation at a region to be observed, a tube voltage and a tube current that are appropriate for a patient has been proposed (for example, Japanese Unexamined Patent Publication No. 2005-149762 or the like). In this method, first, a two-dimensional radiographic image of the patient is obtained, and the transmittance of radiation at the region to be observed, which is in the obtained two-dimensional radiographic image, is used to set the tube voltage and the tube current.
Conventionally, when tomographic images are obtained by tomosynthesis radiography, even if the shapes or sizes of subjects differ from each other, radiography is performed without changing a radiation irradiation angle (photography angle or radiography angle or irradiation angle) at which a subject is irradiated with radiation and the number of times of irradiation in many cases. In tomosynthesis radiography, the subject is irradiated with radiation from various directions, and a distance from a radiation source to the subject differs depending on the radiation irradiation angle. Therefore, a region, of which a clear image is obtained, differs depending on the radiation irradiation angle, at which the subject is irradiated.